Continue performance of an efficacy trial of group A meningococcal polysaccharide vaccine in a pediatric population of age range three months to six years (as in 1974-1975) extending up to eighteen years in 1975-1976. Continue surveillance and sociologic studies of cases, followup of antibody lavels, and nasopharyngeal carrier rates in a sample of vaccinees. Study the evolution of the epidemic especially in regard to possible shift to other serogroups and as correlated with the immune status of the population.